The present invention relates to a variable displacement compressor used in vehicle air conditioners that is driven by a vehicle engine to compress refrigerant gas and changes the pressure in a crank chamber to vary the displacement. More particularly, the present invention pertains to a device for controlling the compressor displacement.
A typical displacement control device for a variable displacement compressor (hereinafter referred to as compressor) includes a control passage. The control passage connects a suction pressure zone with a crank chamber, which accommodates a cam plate. The control device adjusts the pressure in the crank chamber to alter the inclination of the cam plate. The compressor displacement is varied accordingly. The crank chamber is connected to a discharge pressure zone by a supply passage. The supply passage supplies highly pressurized refrigerant gas from the discharge pressure zone to the crank chamber. A displacement control valve is located in the control passage. The opening size of the control valve is changed by a computer, which changes the flow rate of refrigerant gas from the crank chamber to the suction pressure zone. Accordingly, the crank pressure is adjusted.
A typical displacement control valve includes a valve body, a pressure sensing mechanism and an electric actuator. The pressure sensing mechanism moves the valve body in accordance with the pressure of refrigerant gas that is drawn into the compressor (hereinafter referred to as the actual suction pressure). Accordingly, the opening size of the control passage is adjusted. The actuator changes a force acting on the valve body based on the value of a current, which is determined by the computer. A target suction pressure is determined by the value of the current. The target suction pressure is a referential value for the actuation of the pressure sensing mechanism.
When the actual suction pressure exceeds the target suction pressure, the pressure sensing mechanism moves the valve body in a direction increasing the opening size of the control passage. This increases the flow rate of refrigerant gas discharged from the crank chamber to the suction pressure zone and decreases the pressure in the crank chamber. Accordingly, the compressor displacement is increased. When the actual suction pressure drops below the target suction pressure, the pressure sensing mechanism moves the valve body in a direction closing the control passage. This decreases the flow rate of refrigerant gas from the crank chamber to the suction pressure zone, which increases the pressure in the crank chamber. Accordingly, the compressor displacement is decreased.
The electric actuator decreases the target suction pressure, for example, when the value of current from the computer is increased. Thus, the pressure sensing mechanism moves the valve body to adjust the opening size of the control passage for maintaining a lower actual suction pressure. When the value of the current is decreased, the actuator increases the target suction pressure. Therefore, the pressure sensing mechanism moves the valve body to adjust the control passage for maintaining a high actual suction pressure. When the current value is zero, the actuator maximizes the target suction pressure. In this case, the actual suction pressure is significantly lower than the target suction pressure. The pressure sensing mechanism then fully closes the control passage so that the difference between the actual suction pressure and the target suction pressure will be eliminated.
The load on a vehicle engine increases, for example, when the vehicle is rapidly accelerated. In such a case, the above described control device minimizes the compressor displacement to decrease the engine load. This is referred to as a displacement limiting control procedure. As shown in FIG. 7(a), when a vehicle is rapidly accelerated, the computer stops the supply of current to the electric actuator of the displacement control valve. Also, the computer maximizes the target suction pressure as shown in FIG. 7(b) to minimize the compressor displacement. Accordingly, the torque generated by compression reaction of the compressor is decreased as shown in FIG. 7(c).
The displacement limiting control procedure is finished when a predetermined period has elapsed after the procedure was initiated. Thereafter, a normal compressor control procedure, which is performed according to the cooling load, is started. The computer increases the value of the current supplied to the actuator from zero to a value that corresponds to the cooling load. That is, the computer decreases the target suction pressure from the maximum value to a desired value that corresponds, to the cooling load.
However, if the target suction pressure suddenly drops from the maximum value to the desired value, the actual suction pressure cannot follow the sudden change of the target suction pressure. That is, the actual suction pressure is temporarily significantly greater than the target suction pressure. The pressure sensing mechanism quickly increases the opening size of the control passage, which suddenly decreases the pressure in the crank chamber and suddenly increases the compressor displacement. As a result, the torque of the compressor is suddenly increased, which significantly lowers the engine speed and performance.
Therefore, as shown in FIG. 7(a), the computer linearly increases the value of actuator current from zero to a target value with a slope as shown in FIG. 7(a), over a certain period. In other words, the computer linearly decreases the target suction pressure from the maximum value to the desired value over the certain period, as shown in FIG. 7(b). Therefore, when the displacement limiting control procedure is finished, the actual suction pressure does not significantly exceed the target suction pressure. The compressor displacement, or the torque of the compressor, is gradually increased as shown in FIG. 7(c). The engine performance is thus improved.
A uniformly dashed line and a line made up of one long and two short dashes represent changes of the actual suction pressure in two different cases. Although the target suction pressure and the actual suction pressure are the same in the two cases before the displacement limiting control procedure is started, the actual suction pressure differs between the two cases at the end of the displacement limiting control procedure. In the normal control procedure, the target suction pressure is changed in accordance with the demand for cooling by the passengers, which is indicated by a target passenger compartment temperature. Therefore, depending on the target compartment temperature, which is set by a passenger, the target suction pressure may have the same value for different compartment temperatures. Even if the target suction pressure is the same in different performances of the displacement limiting control procedure, the evaporation amount of refrigerant in the evaporator may be different. Accordingly, the actual suction pressure may vary at the end of each displacement limiting control procedure.
For example, if the actual suction pressure increases too slowly as illustrated by the line having one long and two short dashes in FIG. 7(b), the actual suction pressure is significantly lower than the target suction pressure when the displacement limiting control procedure is completed. Therefore, the actual suction pressure surpasses the target suction pressure and the pressure sensing mechanism moves the valve body in a direction to lower the pressure in the crank chamber only when a relatively long time has elapsed after the displacement limiting control procedure is completed. As a result, the compressor displacement is increased from the minimum value only when a long period elapsed after the limiting control procedure is completed, as illustrated by the line having. one long and two short dashes in FIG. 7(c). When the occurs, the compartment temperature may rise significantly from the temperature that existed before the displacement limiting control procedure was started, which disturbs the passengers.
As described above, the control device linearly increases the value of the current supplied to the electric actuator, over the certain period, when switching from the displacement limiting control procedure to the normal control procedure. The slope of the current increase line, see FIG. 7(a), is determined based on the expected value of the actual suction pressure at the end of the displacement limiting control procedure. However, the actual suction pressure may vary when the displacement limiting control procedure is completed and may be different from the expected actual suction pressure. Therefore, when the compressor displacement is restored from the minimum value to a value that corresponds to the cooling load, the restoration characteristics vary.
Accordingly, it is an objective of the present invention to provide a displacement control device for variable displacement compressors that quickly restores a compressor displacement from the minimum value to a value that corresponds to the cooling load regardless of the suction pressure when a temporary displacement control procedure is completed.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a displacement control device for a compressor that draws and compresses refrigerant gas and changes the displacement in accordance with the pressure in a controlled pressure chamber is provided. The device includes a control valve, a detector and a controller. The control valve controls the pressure in the controlled pressure chamber. The control valve has a valve body, a pressure sensing mechanism and an electromagnetic actuator. The pressure sensing mechanism displaces the valve body in accordance with the pressure of refrigerant gas drawn into the compressor. The electromagnetic actuator adjusts a force applied to the valve body based on the value of a current supplied to the electromagnetic actuator, thereby changing a target suction pressure, which is a reference value for the operation of the pressure sensing mechanism. The detector detects the pressure of refrigerant gas that is drawn into the compressor. The controller selects a control procedure from a normal displacement control procedure and a temporary displacement control procedure based on the suction pressure detected by the detector. When the normal displacement control procedure is selected, the controller sends a current, the value of which is based on external information, to the electromagnetic actuator. When the temporary displacement control procedure is selected, the controller sends a current having a specific value to the actuator to maximize the target suction pressure. When switching from the temporary displacement control procedure to the normal displacement control procedure, the controller changes the value of the current supplied to the electromagnetic actuator to a predetermined target value over a predetermined period, and determines the pattern of change of the current to the electromagnetic actuator during the predetermined period based on the suction pressure during or at the end of the temporary displacement control procedure.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.